Shadow mask for use in color picture tube and method for fabricating same

ABSTRACT

A shadow mask for use in the color picture tube and a method of fabricating the same are disclosed in which developement of the stretcher strain phenomenon is prevented by reducing the diameter of crystal grains of iron sheet material used for the shadow mask.

United States Patent 11 1 Yamada et a1.

SHADOW MASK FOR USE IN COLOR PICTURE TUBE AND METHOD FOR FABRICATINGSAME Inventors: Junichi Yamada; Etuzo Terashima;

Akira Takai, all of Mobara Japan Assignee: Hitachi, Ltd., Japan Filed:Aug. 5, 1974 Appl. No.: 494,905

U.S. Cl. l48/l2.l; 96/36; 148/36 Int. Cl. C21D 9/46 Field of Search148/12 R, 12.1; 96/36,

References Cited UNITED STATES PATENTS 3.510.335 5/1970 Mcars ..148/12.1

-- ANNEALING ROLLING Prinmry E.\'aminerW. Stallard [451 Sept. 30, 1975Frantzen 96/36.]

Labuna et al. 96/361 Attorney, Agent, or FirmCraig & Antonelli ABSTRACTA shadow mask for use in the color picture tube and a method offabricating the same are disclosed in which developement of thestretcher strain phenomenon is prevented by reducing the diameter ofcrystal grains of iron sheet material used for the shadow mask.

ANNEALING -/-O SKIN PASS Mb CUTTING IRON SHEET ROLLlNG STEPS L WINDINGSENSITIZER CQATING EXPOSJRE DEVELOPMENT HJRNING H-Ul'O-ETCHING STEPS LETCHING MASK-SHAPING STEPS 4 Claims, 6 Drawing Figures PRESS- SHAPINGBLACKEN [N6 MASK ASSEMBLDGE US. Patent Sept. 30,1975

ANNEALING ROLLING CUTTING WINDING SENSITIZER COATING EXPOSUREDEVELOPMENT BURNING ETCHING Sheet 1 0f 5 MASK-Sl-iAPING STEPS ANNEALINGROLLER LEVELLER PRESS-SHAPING BLACKEN l NG MASK ASSEMBLAGE FIG. 2

TENSILE sTREssa (kg/mm m 04 9 O O I I l I IO 20 3O 4O EXTENSIONAL STRAINe (/o) US. Patent Sept. 30,1975 Sheet 3 of 5 3,909,311

F I G 3 I' I ANNEALING ROLLING ANNEALING a SKIN PASS b ROLLER ILLEVELLER I PRESS- SHAPING BLACKEN l NG EXPOSURE I I S SIEMBLI GEDEVELOPMENT BU RN I NG ETCHING CUTTING WINDING SENSI TIZER COATINGPHOTO-ETCHING STEPS U.S. Patent Sept. 30,1975 Sheet 4 of 5 3,909,311

F I G 4 ANNEALING TEMPERATION (ANNEALING TIME OF IO MINUTES)s'sosboebo'rbomebo HBSINI'IN EIZIS NIVEIS) V\LISV BZIS NIVHQ "IVlSAHQSHADOW MASK FOR USE IN COLOR PICTURE TUBE AND METHOD FOR FABRICATINGSAME The present invention relates to a shadow mask for use in thetelevision picture tube and a method of fabricating the same, or more inparticular to a method of fabricating the shadow mask used in the colorpicture tube in which a superior picture is produced by minimizingdisplacement of shadow mask apertures which otherwise might lead to whatis called the stretcher strain phenomenon.

The prior art to the present invention and the present invention will bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a flow diagram showing steps of fabricating the conventionalshadow mask;

FIG. 2 is a stress-strain curve for the conventional shadow mask;

FIG. 3 is a flow diagram for explaining the Steps of fabricatiang theshadow mask according to the present invention;

FIG. 4 is a diagram showing the relationship between the annealingtemperature and crystal grain size;

FIG. 5 is a diagram showing the relationship between crystal grain sizeand strength; and

FIG. 6 is a diagram showing stressstrain curves before and after a skinpass step.

Generally, the conventional color picture tube of shadow mask typeemploys, of all the magnetic metals, an iron sheet as a shadow maskmaterial for reasons of workability and cost, and the shadow mask of aniron sheet is fabricated according to the steps shown in FIG. 1.

First, a shadow mask material in the form of iron sheet beltapproximately 700 mm wide and approximately 0.3 mm thick is annealed,rolled into the approximate thickness of 0.18 mm by means of a twohightandem rolling mill, cut into the width of about 550 mm and taken up ina roll. The rolled iron sheet belt is cut into a plurality of ironsheets each about 400 kg and transferred to the photoetching step. Inthe pho toetching step, each iron sheet is coated with a sensitizer onboth sides and cut into a plurality of sheets again each about 50 kg.Glass reference patterns preformed with predetermined dots or slots areclosely attached onto both sides of the iron sheet, so that the ironsheet is exposed to ultraviolet ray and developed with a hot-waterspray. After the predetermined dotos and slots have been formed, thesheet is subjected to a burning process at about 320C for about 5minutes to heat and harden the remaining photosensitive film. The sheetis etched by spraying a ferric chloride solution thereby to cutapertures of predetermined size. Thus a flat base sheet of the shadowmask is obtained. This base sheet is annealed for 10 minutes at 940C,smoothed with a roller lcveler and applied through a press to formpredetermined curvatures and side walls. Finally, the shadow masksurface is blackened to form a layer of triferric tetraoxide for thepurpose of anticorrosion.

In the above-described conventional steps of shadow mask fabrication,the process of rolling the iron sheet 0.30 mm thick into the thicknessof 0.18 mm after annealing involves a rolling reduction of as large as40 (the rolling reduction being defined as the ratio of the thicknessreduction of a sheet to the original thickness thereof in a rollingprocess), so that crystal grains of the iron sheet as viewed from thesection thereof in parallel to the direction of rolling arespindle-shaped, very fine and have an elongation of l to 2 Since theelongation of at least 3 is required for the step of pressshaping,however, the iron sheet thus rolled into the thickness of 0.18 mm willbe broken as shown by curve 1 of FIG. 2, if press-shaped without anypreliminary protective measure. As such a protective measure, the stepof annealing is added as a preliminary step for the press-shaping. As aresult of the annealing, the crystal grain diameter in the iron sheetsection, as expressed in ASTM ferrite grain size number, changes to 4 or5 as compared with 10 or 11 for the original sheet material. Thisrepresents an elongation of about 30 which eliminates the risk of theiron sheet being broken during the press-shaping step as will be seenfrom curve 2 in FIG. 2. Even though the larger the crystal graindiameter, the higher the annealing temperature, the annealing time ofmore than 10 minutes is not very effective because of the small weightof the shadow mask base sheet. In other words, in the case of crystalgrain lower than 3, for example, 2 or 1 in ASTM ferrite grain sizenumber, the diameter of the grains is so large that the surface of theshadow mask base sheet roughens for deterioration of its quality.Further, the heat required for prolonged annealing causes excessiveconsumption of furnace materials, thereby adversely affecting economy ofcost. A low annealing temperature, on the other hand, results in lessgrowth of grain diameter in the iron sheet section. Even though it is aknown fact that a roller leveller process following an annealing processreduces the yield point extension as shown by curve 3 of FIG. 2, grainslarger than 5 or 6 in ASTM ferrite grain size number, for example, thoseof 7 or 8, are not reduced below 2 in yield point extension even if theyare applied through a roller leveller repeatedly many times, forexample, 50 times. As a result, during the press-shaping processaccompanied by a maximum elongation of 3 what is called the stretcherstrain phenomenon presents itself conspicously in which the elongationis different at different points of the iron sheet, ranging from 1 to 3thereby leading to the disadvantages of partial lack of uniformity ofboth mask aperture pitches and sizes. A shadow mask with such maskapertures also Iasks uniformity in transmission of electron beams andthe resulting color picture tube is a product so low in commercial valueas to have uneveness in white color and brightness. Specifically, in theevent that the mask apertures are arranged at a pitch of 600 ,u, thedifference of a small 2 p. in pitch causes lack of transmissionuniformity. In fact, this severe rule also supplies to the mask aperturediameter. In the case of mask apertures 200 pt in diameter, for example,an error of 2 ,u. will result in lack of transmission uniformity if itoccurs with several successive apertures.

In the conventional shadow mask comprising an iron sheet with crystalgrains of 4 or 5 in ASTM grain size number, the shadow mask materialbecomes thinner by being melted on one hand and mask apertures areenlarged on the other hand during the well known postetching stop.(Especially, in the shadow mask for use in the conventional colorpicture tube of black matrix type or non-black matrix type, for example,postaccelerlation type, the post etching may be carried out.) Thus thestrength of the shadow mask material is reduced, with the result thatwhen a localized large beam current brightens part of the viewing screenin actuating the color picture tube incorporating such a shadow mask,shadow mask portions about 60 mm inwardly of the periphery thereof,unlike the central and peripheral portions, are often distorted, therebycausing relative displacement of the landing positions of electron beamsonto the screen and phosphor materials and hence the image thereat showsan entirely different color. Specifically, when a beam current of 2 mAflows in an area within 60 mm in diameter at the portions in question, adisplacement of about 140 t occurs in electron beam landing on thescreen in the case of a inches black matrix color picture tube of 110deflection type.

The object of the present invention is to provide a shadow mask for usein a color picture tube comprising an iron sheet material whose grainsize is reduced without giving rise to any stretcher strain phenomenonand a method of fabricating the same.

According to one aspect of the invention, there is provided an improvedmethod of fabricating a shadow mask for use in a color picture tubecomprising the processes of rolling, photo-etching and mask-shaping,characterized in that the rolling process includes a first annealingstep for annealing a shadow mask material, a rolling step for rollingthe annealed material into a desired thickness, a second annealing stepfor annealing the rolled material, and a skin-pass step for skin-passingthe material applied through the second annealing step.

On embodiment of the present invention will be described more in detailbelow.

In FIG. 3 showing a flow diagram for explaining the fabricating stepsaccording to an embodiment of the invention, those sections which areidentical to those shown in FIG. 1 have the same functions as those ofthe corresponding sections in FIG. 1; and it should be noted that theannealing step after etching must not be adopted and the roller levellerstep may be eliminated from the present invention.

In embodying the present invention, as a preparatory step for theconventional press shaping step, a second annealing step and a skin-passstep as shown by a and b respectively in FIG. 3 are inserted between thestep of rolling the iron sheet into thickness, say, 0.18 mm and the stepof cutting it to width, say, to 543 mm. The annealing step a isaccomplished at a temperature from 550 to 750C or preferably from 660Cto 700C. The annealing of the iron sheet at this temperature for about10 minutes causes the spindle-shaped crystal grains rolled into thethickness of 0.18 mm to be recrystallized and finally settle, and aswill be seen from FIG. 4 showing results of an experiment conducted onthe relationship between annealing temperatures and grain size, theychange to 7 or 8 in ASTM grain size number. It will also be seen fromFIG. 5 showing the relationship between grain size and strength thatcrystal grains with ASTM grain number of 7 or 8 have a high strength andthe surface of the iron sheet is not uneven.

The skin pass step b, on the other hand, involves a rolling reduction of0.5 to 5 an optimum reduction being 1.5 to 2.0 The slight rollingeffected by the skin-pass process eliminates the yield point extension4a of the stress-strain curve 4 prior to the skin-pass process as shownin FIG. 6, so that the extension becomes about 45 after the skin-passprocess as shown by the stress-strain curve 5, thereby preventing anycase of breakage during the press-shaping step. It is important tofollow the annealing step a and skin-pass step b in this order. If thepress-shaping step is entered immediately after the annealing step a,for example, the stretcher strain phenomenon is not eliminated; nor isit possible to do away with it during the roller leveller step. On theother hand, the rolling reduction in a skinpass as high as 10 makes sohard a surface of the iron sheet that there is too great an amount ofspring-back after the press-shaping step making it impossible to obtaina predetermined curvature. Again, therefore, the optimum rollingreduction in skin-pass is 1.5 to 2.0

In addition to the black matrix tube of post-etching type as mentionedabove, the present invention is applicable with equal effect to thoseblack matrix tubes in which post-etching is not effected but a lightsource is appropriately operated to form graphite holes small indiameter by the use of a shadow mask with apertures large in diameter aswell as to color picture tubes other than black matrix type. That is tosay, in these addi tional fields of applications where the thickness ofthe shadow mask is not reduced nor aperture diameter enlarged duringcolor picture tube fabrication steps, it is possible to achieve the samehigh strength of crystal grains small in size as in the presentinvention, thereby making a reduction in quality due to shadow maskdistortion a rarity. Furthermore, in spite of the fact that the steps offabricating color picture tubes with a shaping mask involve three andfour applications thereof through the baking furnace and exhausingfurnace, the size of grains in the shadow mask section remains unchanged.

It will be apparent from the foregoing description that according to thepresent invention a shadow mask is made of an iron sheet with grainshaving a size 7 or 8 in ASTM grain size number in a shadow mask section,making possible a high-quality color picture tube with stablecharacteristics which develops no distortion.

What is claimed is:

1. In a method of fabricating a shadow mask for use in a color picturetube comprising the processes of rolling, photo-etching andmask-shaping; the improvement of said rolling process comprising a firstannealing step for annealing a shadow mask material, a rolling step forrolling said annealed material into a desired thickness, a secondannealing step for annealing said rolled material, and a skin-pass stepfor skin-passing said material after said second annealing step.

2. The method of fabricating a shadow mask according to claim 1, inwhich said second annealing step is effected at a temperature in therange from 600C to 700C to achieve ASTM ferrite grain size number 7 to 8of the crystal grains of said shadow mask material.

3. The method of fabricating a shadow mask according to claim 1, inwhich said second rolling step involves a rolling reduction of 1.5 to2.0

4. A shadow mask for use in a color picture tube, made by the method ofclaim 1 in which the grain size in a given section of an iron sheet usedas a material of said shadow mask is 7 to 8 in ASTM ferrite grain sizenumber.

1. IN A METHOD OF FABRICATING A SHADOW MASK FOR USE IN A COLOR PICTURETUBE COMPRISING THE PROCESSES OF ROLLING, PHOTOETCHING AND MASK-SHAPINGTHE IMPROVEMENT OF SAID ROLLING PROCESS COMPRISING A FIRST ANNEALINGSTEP FOR ANNEALING A SHADOW MASK MATERIAL, A ROLLING FOR ROLLING SAIDANNEALED MATERIAL INTO A DESIRED THICKNESS, A SECOND ANNEALING STEP FORANNEALING SAID ROLLED MATERIAL AND A SKIN-PASS STEP FOR SKINPASSING SAIDMATERIAL AFTER SAID SECOND ANNEALING STEP.
 2. The method of fabricatinga shadow mask according to claim 1, in which said second annealing stepis effected at a temperature in the range from 600*C to 700*C to achieveASTM ferrite grain size number 7 to 8 of the crystal grains of saidshadow mask material.
 3. The method of fabricating a shadow maskaccording to claim 1, in which said second rolling step involves arolling reduction of 1.5 % to 2.0 %.
 4. A shadow mask for use in a colorpicture tube, made by the method of claim 1 in which the grain size in agiven section of an iron sheet used as a material of said shadow mask is7 to 8 in ASTM ferrite grain size number.